1. Field of the Invention
The present invention relates to a method of forming a resist pattern in which a resist pattern is formed by a double exposure process.
Priority is claimed on Japanese Patent Application No. 2008-203201, filed Aug. 6, 2008, the content of which is incorporated herein by reference.
2. Description of Related Art
Techniques (pattern-forming techniques) in which a fine pattern is formed on top of a substrate, and a lower layer beneath that pattern is then fabricated by conducting etching with this pattern as a mask are widely used in the semiconductor industry for IC fabrication and the like, and are attracting considerable attention.
These types of fine patterns are usually formed from an organic material, and are formed, for example, using a lithography method or a nanoimprint method or the like. In lithography techniques, for example, a resist film composed of a resist composition that contains a base material component such as a resin is formed on top of a supporting material such as a substrate, and the resist film is subjected to selective exposure of radial rays such as light or electron beam through a photomask in which a pattern (mask pattern) with a predetermined shape has been formed, followed by a developing treatment, thereby forming a resist pattern having a predetermined shape on the resist film. A resist composition in which the exposed portions become soluble in a developing solution is called a positive-type, and a resist composition in which the exposed portions become insoluble in a developing solution is called a negative-type. Using this resist pattern as a mask, a semiconductor or the like is produced by conducting a step in which the substrate is processed by etching.
In recent years, advances in lithography techniques have led to rapid progress in the field of pattern miniaturization.
Typically, these miniaturization techniques involve shortening the wavelength of the exposure light source. Conventionally, ultraviolet radiation typified by g-line and i-line radiation has been used, but nowadays KrF excimer lasers and ArF excimer lasers are now starting to be introduced in mass production. Furthermore, for further improving the resolution, research is also being conducted into lithography techniques that use an exposure light source having a wavelength shorter than these excimer lasers, such as F2 excimer lasers, electron beam, extreme ultraviolet radiation (EUV), and X ray.
Resist compositions for use with these types of exposure light sources require lithography properties such as a high resolution capable of reproducing patterns of minute dimensions, and a high level of sensitivity to these types of exposure light sources. As a resist composition which satisfies these conditions, a chemically amplified resist composition is used, which includes a base material component that exhibits a changed solubility in an alkali developing solution under action of acid and an acid generator that generates acid upon exposure (for example, refer to Patent Document 1). For example, a chemically amplified positive resist typically contains, as a base material component, a resin which exhibits increased solubility in an alkali developing solution under the action of acid. In the formation of a resist pattern, when acid is generated from the acid generator upon exposure, the exposed portions become soluble in an alkali developing solution.
Further, as one of the techniques to further improve the resolution, super-resolution techniques have also been studied, such as a phase shift method in which a phase shift mask is used as a photomask (for example, refer to Non-Patent Document 1), and a modified illumination method (for example, refer to Non-Patent Document 2) in which a modified illumination system such as a dipole illumination system is employed as illumination conditions during exposure.
On the other hand, in the production of dynamic random access memory (DRAM) or flash memory, a pattern in which holes are regularly arranged (hereafter, referred to as “contact hole pattern”) is formed. As higher packing densities and further miniaturization are required for cell structures, there is an increase in the demand for techniques which enable formation of finer contact hole patterns at higher densities.
Conventionally, chemically amplified positive resist compositions have been widely used for formation of fine contact hole patterns. This is because when a chemically amplified negative resist composition is used, the mask structure becomes complex, and contrast satisfactory for forming images is difficult to achieve, and properties of the formed patterns are inferior to those obtained when a chemically amplified positive resist composition is used, and thus the use of a chemically amplified negative resist composition for forming contact hole patterns is not common.
However, when forming a contact hole pattern by subjecting a resist film formed of a chemically amplified positive resist composition to exposure through a photomask, as compared to the case where a line and space pattern is formed, incident energy of light to the resist film cannot be optically increased, and thus there is a limit for improving the resolution. That is, when the resolution limits of resist patterns are compared using Rayleigh's formula (resolution limit=k1×(wavelength of light source)/(lens aperture)), k1 values for line and space patterns fall within a range between 0.25 and 0.3, whereas k1 values for hole patterns fall within a range between 0.4 and 0.5. In other words, regardless of the wavelength of exposure light source or lens aperture, the resolution limit for hole patterns cannot match the level of resolution limit achieved for line and space patterns.
Further, although reducing the distance between individual holes is important for enhancing packing densities of contact hole patterns, it is difficult to shorten the distance with the method described above.
Furthermore, a problem also arises in that roughness can develop on the upper surface and side wall surfaces of the formed pattern. For example, roughness on the side wall surfaces of a pattern, so-called line edge roughness (LER), can cause various defects such as distortions around the holes in hole patterns, and consequently has the potential to adversely affect the formation of very fine semiconductor elements. This problem of roughness becomes more significant as the pattern dimensions are reduced.
A double patterning process is one of the techniques which is capable of forming contact hole patterns of high resolution.
There are several different types of double patterning process and for forming contact hole patterns, a double patterning process in which a lithography step (from application of resist compositions to exposure and developing) and an etching step are repeated twice or more to form a pattern can be used (for example, refer to Non-Patent Document 3). Formation of contact hole patterns according to such a double patterning process can be conducted, for example, by the procedure shown in FIGS. 5A to 5F.
In other words, as shown in FIG. 5A, a laminate is prepared in which a substrate 101, a lower-layer film 102 and a hard mask 103 are laminated. Subsequently, a resist film is formed on the hard mask 103, and the resist film is subjected to selective exposure and developing through a photomask 105, as shown in FIG. 5B, thereby forming a resist pattern 104 in which a plurality of hole patterns are arranged with a hole diameter of d/4 and a pitch d.
Then, the resist pattern 104 is subjected to etching by using the hard mask 103 as a mask, followed by the removal of the remaining resist pattern 104. As a result, as shown in FIG. 5C, a hard mask 103′ is obtained in which a resist pattern is transferred.
Subsequently, as shown in FIG. 5D, the position of the photomask 105 is shifted, and a resist composition is applied on the hard mask 103′. As a result, a resist film is formed which fills in the gaps within the hard mask 103′ and which has a greater film thickness than that of the hard mask 103′. Then, the resist film is subjected to selective exposure through the shifted photomask 105, followed by developing to form a resist pattern 106.
Then, the resist pattern 106 is subjected to etching by using the hard mask 103′ as a mask, followed by the removal of the remaining resist pattern 106. As a result, as shown in FIG. 5E, a hard mask 103″ is obtained in which a pattern is transferred, the pattern formed of a plurality of hole patterns arranged with a hole diameter of d/4 and a pitch d/2.
The pattern of the hard mask 103″ is transferred to the lower-layer film 102 by conducting an etching process using the hard mask 103″ as a mask. As a result, as shown in FIG. 5F, a pattern 102′ is formed which has a pitch as large as half the pitch of the photomask 105 used.
As described above, according to the double patterning process, a resist pattern with a higher level of resolution can be formed, as compared to the case where a resist pattern is formed by a single lithography step (namely, a single patterning process), even when a light source with the same exposure wavelength is used, or even when the same resist composition is used. Furthermore, the double patterning process can be conducted using a conventional exposure apparatus.
However, in the conventional double patterning process, formation of a lower-layer film on a substrate is usually required, and each of patterning of a resist film and etching of a hard mask layer below needs to be conducted at least twice, which results in the increase of number of steps required and amount of chemical drugs used, and ultimately the increase in the production cost.
As one of the double patterning processes, a double exposure technique is also proposed, in which exposure is conducted twice or more after formation of a resist film, followed by developing to form a resist pattern (for example, refer to Patent Documents 2 and 3). Like the double patterning process described above, this type of double exposure technique is also capable of forming a resist pattern with a high level of resolution, and also has an advantage in that fewer number of steps is required, as compared to the above-mentioned double patterning process.
However, when contact hole patterns are formed by conventional techniques involving multiple exposure, various problems such as an overlay problem arise, and thus formation of fine and high-density contact hole patterns has been impossible.
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2003-241385
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2008-098231
[Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2008-159874
[Non-Patent Document 1] Principles of Lithography (2nd Edition), Harry J. Levinson, SPIE Press, pp. 295-307.
[Non-Patent Document 2] Principles of Lithography (2nd Edition), Harry J. Levinson, SPIE Press, pp. 273-282.
[Non-Patent Document 3] Proceedings of SPIE (U.S.), vol. 5256, pp. 985-994 (2003)